This invention relates to a lead free solder structure for the assembly of electronic components, and more particularly, to a lead free solder hierarchy for use in a second level solder connection in electronic components such as joining an electronic module to a circuit card or board.
The use of solder to join components in an electronic assembly is well known in the art. There are a variety of electronic components which require connection to other electronic components or to other levels of packaging. Typical examples are second level surface mount technology where column grid arrays (CGA) or ball grid arrays (BGA) are used to form an interconnection between a circuit board and an electronic module assembly such as a semiconductor chip attached to a multilayer ceramic (MLC) substrate.
The module assemblies are connected to the board or card by joining solder balls or columns to the substrate bottom surface metallurgy (BSM). The module is subsequently joined to the board or card in what is referred to as a second level assembly.
For various environmental reasons the industry is moving to a lead free solder strategy for component assembly. The current lead free interconnect solution is to use Tin/Silver/Copper eutectic as the joining alloy for both the module and card side joints. This alloy is used because most of the higher melting alloys have other problems associated with them. For example, Tin Antimony (235-240° C.) has environmental issues regarding oxides of antimony.
The current solution to a lead free system is to use the same solder alloy (Sn/Ag/Cu of a specific composition, such as 3.8Ag, 0.7Cu, balance Sn) on both the module and card or board side column joints. This has two significant problems associated with it. First is that the column may tilt or collapse during second level assembly because the fillet on the module side will undergo melting. Joining of Cu columns with Sn, Ag and Cu (SAC) on the module bottom surface metallurgy BSM pad and SAC paste on the card side causes modules to fall over during joining at 235° C. SAC eutectic melts at 217° C. Second, the card rework process becomes much more involved because during module removal from the card almost all of the columns will remain on the card and then have to be painstakingly removed prior to rejoin. Rework of modules from the card would leave many columns behind on the card. This is extremely messy and time consuming in manufacturing production. Another problem is that no readily available alloy exists with a melting temperature between 220° C. and 260° C. that does not have associated environmental, process or reliability issues.
The present invention solves these problems by creating a solder melting hierarchy for second level assembly. The present invention provides an off-eutectic concentration of SnCu or SnAg that adds sufficient intermetallics to the module BSM fillet to satisfy the following requirements: The material must reflow and wet at the standard joining temperatures from 235° C. to 260° C. to satisfy all lead free solder applications. The material must stabilize the module BSM joint to prevent modules from falling over during joining to the card or circuit board. The material must remain cohesive during card or board rework to insure as many columns as possible stay attached to the module and are not left behind on the card or board.
The present invention discloses the use of off-eutectic lead free solder alloys containing two or more of the following: tin, silver, copper. These off-eutectic compositions contain secondary intermetallic phases which melt at a temperature above 280° C. and add substantial structural support to the module BSM joint. The present invention achieves the desired temperature hierarchy in the module/Column BSM joint which allows for card or board attach without the assistance of a mechanical standoff and module removal/rework from the card or board whereby the majority of columns remain attached to the module BSM. This eliminates the problems involved when the same solder is used on both the module side and card side pads. Without a solder temperature hierarchy, all or most of the columns remain on the card or board through rework creating a very labor intensive exercise in preparing the site for the rejoin of a new module. The invention is the use of different alloys to create a stable structure during initial card join and subsequent module/card rework to prevent column collapse.
This material solution can also replace the single melt Sn, Ag and Cu (SAC) solder ball planned for lead free BGA. These and other purposes of the present invention will become more apparent after referring to the following description considered in conjunction with the accompanying drawings.